A “bump” is a three-dimensional shape (half sphere or rectangular) salient, made of solder or gold located on the face of a microelectronic chip. The bump exists in some chips and substitutes as leads by means of which the component is connected to the printed circuit when the bumps are soldered to the board. The bump shape is usually a half ball alike. A chip can contain a large number of bumps, which should be of the same height in order to connect all of them to the board at the same time. Actually, there are differences between the heights of the various bumps, as a result of the production process. Only small height differences can be allowed, and these must be within the tolerance limits. Therefore the height of each bump must be checked.
Various systems for wafer-inspection are known and the main disadvantage of all those systems is a low accuracy in height measurement. Therefore, the main object of the present invention is that it uses a Confocal Height Measuring System (CHMS) to achieve accurate height measurement.
Confocal Height Measuring System (CHMS) is assembled from a confocal imaging optical setup with chromatic aberration focusing lens, a light source, an optic head that separates the light source to its basic wavelength and a spectrometer.
The confocal imaging optical setup is an optical setup for imaging a point of light source into a sharply focused second point and then reversing the image from the second point onto a tiny spatial filter. Such an optical setup is absolutely blind for all the space except for the sharply focused second point. Field extension can be obtained by stretching the chromatic aberration of the focusing lens of the setup. The new setup, with such a lens, is assembling of infinity of purely confocal systems, one for each wavelength.
Since each wavelength has a different focus length, said setup can be used as height-measuring device to measure the height of a surface point. A white light beam is separated to its basic wavelength beams by the optic head and each beam illuminates the surface. The illumination is reflected back through the confocal imaging optical setup to the spectrometer. Only one wavelength is passed the confocal imaging optical setup, according to height of the surface, which matches the focus length. The wavelength is detected by the spectrometer and translated to the height of the surface point according to a calibration table.
Confocal imaging optical setup and confocal height measuring system are described in patent application, with a French title: “Dispositif de microstratigraphie optique”—national registration number: FR9510401 and publication number: 2 738 343 and patent application, with a French title: “Dispositif de tomographie optique en champ coloré”—national registration number: FR9402489 and publication number: 2 716 727.
Since the low accuracy—in height measurement—is the main disadvantage of the known systems, there is a recognized need for, and it would be highly advantageous to have, a system and a method for wafer-inspection that uses confocal height measuring system for checking the accurate height of bumps for comparing bumps on a wafer.